1. Field of Invention
The present invention relates generally to the field of wireless communications. More particularly, in one exemplary aspect, the present invention is directed to cooperative operation of a base station network (or even multiple heterogeneous networks), wherein base station timing, preceding methods, and/or simplified communication techniques are leveraged to improve spectral efficiency (including time-frequency resource reuse).
2. Description of Related Technology
Universal Mobile Telecommunications System (UMTS) is an exemplary implementation of a “third-generation” or “3G” cellular telephone technology. The UMTS standard is specified by a collaborative body referred to as the 3rd Generation Partnership Project (3GPP). The 3GPP has adopted UMTS as a 3G cellular radio system targeted for, inter alia, European markets, in response to requirements set forth by the International Telecommunications Union (ITU). The ITU standardizes and regulates international radio and telecommunications. Enhancements to UMTS will support future evolution to fourth generation (4G) technology.
A current topic of interest is the further development of UMTS towards a mobile radio communication system optimized for packet data transmission through improved system capacity and spectral efficiency. In the context of 3GPP, the activities in this regard are summarized under the general term “LTE” (for Long Term Evolution). The aim is, among others, to increase the maximum net transmission rate significantly in future, namely to speeds on the order of 300 Mbps in the downlink transmission direction and 75 Mbps in the uplink transmission direction.
The current LTE specification describes several multiple access methods. For the downlink transmission direction, OFDMA (Orthogonal Frequency Division Multiple Access) in combination with TDMA (Time Division Multiple Access) will be used. Uplink data transmission is based on SC-FDMA (Single Carrier Frequency Division Multiple Access) in combination with TDMA. Further, LTE is expected to support full-duplex FDD, half-duplex FDD and TDD (time division duplexing).
Basic Multiple Access Methods
FIGS. 1A-1D further illustrate basic multiple access methods well understood in the wireless transmission arts. In these Figures, it will be recognized that time increases in the direction of a time axis (t), and frequency increases in the direction of a frequency axis (F).
FIG. 1A comprises a first time-frequency diagram illustrating a TDMA (time division multiple access) system. In TDMA, each mobile radio terminal may use the whole frequency band provided for the usage by the mobile radio terminals. However, for each mobile radio device, only a predefined transmission time interval (TTI) is allocated in which the mobile radio device may send and receive useful data. During a transmission time interval 102, only one mobile radio device is active in a radio cell. In TDMA, the number of users that the network can support is equal to the number of TTIs or time slots that are available. This can produce a hard limit (or so-called “non-graceful” degradation), since when the number of users exceeds the number of slots, the system becomes overloaded and the added user cannot be serviced.
FIG. 1B comprises a second time-frequency diagram illustrating a FDMA (frequency division multiple access) system. In FDMA, each mobile radio device may freely use the time domain, but only a predefined (narrow) frequency band 104 within the entire frequency band available for sending and receiving useful data. In the narrow frequency band, only one mobile radio device is active in the radio cell at any given time. In FDMA, the number of users that the network can support is equal to the number of frequency bands which are available throughout a given frequency spectrum, and hence FDMA networks experience service limitations similar to those described above for TDMA systems.
FIG. 1C comprises a third time-frequency diagram illustrating a CDMA (code division multiple access) system. In CDMA (a sub-species of so-called “direct sequence” or DS systems), each mobile radio terminal may send and receive useful data during any time period, and may use the entire available frequency band. In order to avoid interference between the data sent by different senders, each mobile radio device is allocated a binary (pn or “pseudo-noise) code pattern 106. The code patterns which are allocated to the different mobile radio terminals are ideally orthogonal, and data sent by a mobile radio terminal or to be received by the mobile radio terminal is coded (“spread”) by the code pattern allocated to the mobile radio terminal. In CDMA, the number of users that the network can support is directly related to the number of orthogonal spreading codes which are available; hence, a more “graceful” degradation of service is experienced as the network reaches or exceeds capacity. CDMA has the property that the higher the number of users on the network, the more likely the users will interfere with one another. Accordingly, code distribution and power control are critical.
In certain modes of CDMA operation, variable data rates may be supported by assigning various length spreading codes (a high data rate stream requires a shorter length spreading code, which also limits the number of orthogonal codes available to other users).
FIG. 1D illustrates OFDMA (orthogonal frequency division multiple access), which is a special case of FDMA. OFDMA is a multiple carrier method in which the entire frequency band having a bandwidth B is subdivided into M orthogonal sub-carriers 108. Thus, there are M (narrow) frequency bands each with a bandwidth of F=B/M. In OFDMA, a data stream to be sent is divided over a multiplicity of sub-carriers, and is transmitted (generally) in parallel. The data rate of each sub-carrier is accordingly lower than the overall data rate. For each mobile radio terminal, a defined number of sub-carriers are allocated for data transmission. For OFDMA, the number of maximum users that the network can support is the multiple of the orthogonal sub-carriers multiplied by the number of available transmission time intervals. A chief advantage of OFDMA's flexible time-frequency resource allocation, over e.g., CDMA's flexible code allocation, is a higher spectral efficiency (i.e., more bits per unit time per unit of frequency bandwidth).
Existing Wireless Systems and Interference Reduction Approaches
FIG. 2 illustrates a typical cellular system 200. The Access Network comprises a plurality of base station towers 202 that are set at fixed geographic locations (although some networks do utilize moveable base stations or femtocells). Their wireless coverage is indicated by the dotted areas 204. A Core Network (which is not shown) includes a number of different components and logical entities, and governs the operation of the base stations. A mobile station 206 is being served by one of the base stations and can switch between them via a well-known handoff procedure. The base stations are connected in point-to-point communications to facilitate network management. In the exemplary case of a 3GPP LTE (Evolved UMTS) system, eNodeBs (i.e. base stations) serve UEs (i.e. mobile stations). An X2 interface connects the eNodeBs of the E-UTRAN (Evolved UMTS Radio Access Network). The X2 interface is a pre-defined communication pathway between adjacent eNodeBs to assist in handoffs and Radio Resource Management (RRM).
FIG. 2A illustrates one exemplary OFDMA cellular system 200 useful in implementing a 3GPP LTE compliant cellular network, comprising 3 base stations (BS) 202A, 202B, and 202C having wireless coverage ranges of 204A, 204B and 204C, respectively. As shown, BS 202A is transmitting to UE 206A, BS 202B is transmitting to UE 202B, and BS 202C is transmitting to UE 206C. Unfortunately, UE 202A is operating in a region of transmission overlap, and is thus receiving transmissions intended for UE 202B and UE 202C. This unintentional transmission noise due to base station overlap is commonly referred to as Inter-Cell Interference (ICI). Unlike other forms of truly random noise (e.g., Additive White Gaussian Noise or AWGN caused by thermal noise, etc.), ICI is predictable and deterministic. Therefore, improved methods for managing and obviating ICI for cellular networks are desired.
Several solutions have been contemplated to reduce various types of Inter-Cell Interference. For example, U.S. Pat. No. 6,047,165 to Wright et al. issued Apr. 4, 2000 and entitled “Wireless, frequency-agile spread spectrum ground link-based aircraft data communication system” discloses a flight information communication system having a plurality of RF direct sequence spread spectrum ground data links that link respective aircraft-resident subsystems, in each of which a copy of its flight performance data is stored, with airport-located subsystems. To mitigate interference, a frequency management scheme is employed which initially determines the optimum operating frequency for the GDL link, and automatically changes to a better quality frequency channel when the currently established channel suffers an impairment.
U.S. Pat. No. 6,714,775 to Miller issued Mar. 30, 2004 and entitled “Interference canceller” discloses an interference canceller, wherein the interference canceller includes an input receiving an input composite signal, a reference circuit, and a cancellation circuit. The interference canceller derives its own reference signal. Internal derivation of the reference signal is accomplished by suppressing the desired signal included within a sampled quantum of a composite signal. The resultant reference signal is a likeness of the interfering signals; containing substantially only the interference signal. The internally produced reference signal is amplitude and phase adjusted in a time-continuous fashion and summed with the composite signal in the cancellation circuit. The output of the cancellation circuit contains the desired signal and a substantially suppressed interference signal.
U.S. Pat. No. 7,023,938 to Kapoor et al. issued Apr. 4, 2006 and entitled “Receiver for discrete multi-tone modulated signals having window function” discloses a receiver for improving the performance of conventional Discrete Multi-tone Modulation (DMT) based Asymmetric Digital Subscriber Line (ADSL) modems, in the presence of noise and/or interference. A demodulator having an FFT followed by a single-tap-per-bin frequency-domain equalizer is augmented by an additional data-path utilizing windowing or pulse shaping. Windowing is done independently for each symbol over the orthogonality interval and efficiently in the time domain or frequency domain. A decision feedback equalizer at the output of the windowed data-path cancels inter-bin-interference created by windowing.
United States Patent Publication No. 20020002063 to Miyamoto et al. published Jan. 3, 2002 and entitled “Base station control equipment, radio base station equipment, radio terminal equipment, and mobile communication system” discloses base station control equipment, radio base station equipment and radio terminal equipment that together constitute a mobile communication system. These base station control equipment, radio base station equipment and radio terminal equipment update transmitting power of a radio channel allotted to a new visit-zone to a greater and suitable value in time sequence. Therefore, a mobile communication system can keep speech quality of a completed call and transmission quality at higher levels, can improve the number of radio channels that can be formed in parallel in a common frequency band (system capacity) or an information content of information that can be transmitted in parallel with desired transmission quality, and can improve utilization efficiency of a radio frequency.
United States Patent Publication No. 20040022210 to Frank et al. published Feb. 5, 2004 and entitled “Cooperative transceiving between wireless interface devices of a host device” discloses a method and/or apparatus for cooperative transceiving between wireless interface devices of a host device that includes processing that begins by providing an indication of receiving an inbound packet from one wireless interface device (e.g., Bluetooth compliant radio transceiver, IEEE 802.11 compliant radio transceiver, etc.) to another. The wireless interface device receiving the indication processes the indication and, based on the processing, transmits an outbound packet in accordance with the processing of the indication. For example, the wireless interface device receiving the indication may delay transmission until the other wireless interface device has received the packet, or, if transmission of the packet would not interfere with the receiving of the packet by the other wireless interface device, the wireless interface device receiving the indication would transmit its packet.
United States Patent Publication No. 20070155336 to Nam et al. published Jul. 5, 2007 and entitled “Apparatus and method for eliminating multi-user interference” discloses an apparatus and method for eliminating multi-user interference in a codebook-based beamforming system. A transmitter for providing a service to multi-users in the codebook-based beamforming system includes a beamformer for generating beamformed user signals by multiplying transmit data of users, to whom the service is to be provided, by corresponding weighting factor vectors using feedback information; a null space generator for generating a null space matrix orthogonal to weighting factor vectors of other users; and a projector for projecting the beamformed user signals on the corresponding null space matrix and transmitting the resulting signals through a plurality of antennas. Because the multi-user signals can maintain orthogonality, the performance degradation caused by the multi-user interference can be prevented.
WIPO Publication No. WO/2007/021153 to Lee et al. published Feb. 22, 2007 and entitled “Virtual multiple antenna method for OFDM system and OFDM-based cellular system” discloses a virtual multi-antenna method for an orthogonal frequency division multiplexing (OFDM) system and an OFDM-based cellular system. The virtual multi-antenna method includes grouping sub-carriers in a frequency domain of an OFDM symbol and generating at least one group including G sub-carriers; and regarding the G sub-carriers included in the at least one group as multiple channels used in a multi-antenna technique and virtually applying the multi-antenna technique to the transmission and reception of the OFDM symbol. The virtual multi-antenna method can effectively reduce an interference signal and obtain the effects of a spatial division multiple access (SDMA) technique without physically using multiple antennas.
WIPO Publication No. WO/2007/075107 to Taubin et al. published Jul. 5, 2007 and entitled “Autoregressive moving average modeling for feedforward and feedback Tomlinson-Harashima precoder filters” discloses apparatus and methods for providing a Tomlinson-Harashima precoder scheme in which a feedback filter may be constructed to match an approximated feedforward filter, where the feedforward filter is approximated using autoregressive moving average modeling.
WIPO Publication No. WO/2007/037715 entitled “Precoder Design For Different Channel Lengths” discloses a precoder e.g. Tomlinson-Harashima precoder, parameters constructing method, that involves forming set of values of precoder constructions for preset channel length, where values are obtained from applying transmission quality criterion. The method involves forming a set of values for a set of precoder constructions for a predetermined channel length. The values are obtained from applying a transmission quality criterion to each precoder construction for varying channel lengths, where a transmission quality criterion includes using a mean square error at a feed forward filter output of a precoder e.g. Tomlinson-Harashima precoder. The precoder constructions are generated to cover a distance range, where the distance range is divided into a length equal to predetermined channel lengths.
While the foregoing prior art techniques for reducing the effects of ICI disclose careful frequency-planning, the usage of available frequency bands is organized such that neighboring BSs are not concurrently using the same frequency bands. This may require a corresponding limitation on the sub-carrier allocation in an OFDMA-based system (such as 3GPP LTE, WiMAX, etc.). Other possible methods for frequency planning and reuse may require frequency hopping pattern definitions, or other similar limiting or avoidance techniques. One salient drawback associated with such frequency planning techniques is reduced spectral inefficiency. Specifically, each BS must only operate within a fraction of its available band at any given time, in order to minimize neighboring ICI effects. Accordingly, a desirable solution would enable BS operation within its entire available spectrum, while still ensuring that the interference imparted to neighboring cells is minimized.
CIR and “Precoding”
Alternatively, other prior art techniques may compensate to some degree for certain forms of interference to enable limited suppression of corruption, and suffer from the disability of inter alia that channel characteristics are often imperfectly known. Recall that transmission corruption is introduced by unwanted channel noise, of which some causes are random (e.g. thermal noise, weather induced fading, etc.) and others are predictable (such as ICI, and self-induced interference). One characterization of the effects of channel corruption is termed Channel Impulse Response (CIR). A CIR represents the effects on an “imaginary” impulse transmitted from one or more transmitters in a “vacuum”, and their corresponding measured incident response at the receiver antenna. FIG. 2B illustrates an imaginary BS 202, which is the superposition of three (3) “real” BS 202A, 202B, and 202C. Each BS (202A, 202B, 202C) has a corresponding CIR at the UE. However, the aggregate CIR 208 is actually perceived at UE 206, as the UE cannot distinguish which CIR belongs to which of the “real” base stations.
In these interference suppression schemes, UEs are expected to apply interference reduction algorithms to remove ICI. FIG. 3 illustrates one typical method of implementing interference reduction at the UE. The UE estimates the interfering signals 302, and then reconstructs a “guessed” interfering signal and subtracts it from the received signal 304. If the UE has multiple interfering signals, it iterates through each “guessed” signal. Once all the interfering signals have been estimated and removed, the desired useful signal is decoded in step 306. UE-based interference suppression techniques generally work well if the UE is able to obtain correct estimates of the interfering signals, and accurate CIR models.
In practice, the UE must estimate the channel corruption based on its received signal. The UE is never given a “golden” or error-free starting point from which it is to deduce channel corruption. Therefore, estimation errors commonly occur during initial estimates of the CIR, and are propagated throughout subsequent calculations. If the UE has misestimated the level or type of interference, then the suppression stage 304 may not reduce the overall interference level. Furthermore, with large enough estimation errors, the suppression stage may actually degrade performance. Thus, an ideal solution would minimize potentially fallible estimation techniques to reduce interference.
In one alternative prior art solution, interference suppression techniques may be performed at the BS. Such approaches include so-called “Tomlinson-Harashima Precoding (THP)” techniques described in, inter alia, IEEE International Conference Volume 3 to Kusume et al. published May 2005 and entitled “Efficient Tomlinson-Harashima precoding for spatial multiplexing on flat MIMO channel”, and ITG Conference on Source and Channel Coding to Fischer et al. published January 2002 entitled “Space-Time Transmission using Tomlinson-Harashima Precoding”, each of the foregoing being incorporated herein by reference in its entirety.
Unfortunately, extant THP methods have several stringent requirements including: (i) that each BS must know the CIR of all radio channels (i.e., CIRs from all BS to all UEs), and (ii) that each BS must know all the data that the other BS will transmit in the future. Further, each BS must be perfectly synchronized. If all of these requirements are satisfied, then THP can reshape the transmitted signals such that, at a given UE location, the unwanted noise destructively interferes with itself, creating a noise-free channel. Unlike UE-based suppression schemes, each THP BS perfectly “knows” the actual transmitted signal, and does not introduce error from signal estimation. Unfortunately, the aforementioned “omniscient” requirements for THP BS are prohibitive for current practical implementations; this is especially true and burdensome with regard to knowing in advance the data to be transmitted (in effect requiring a voice call, video streaming, etc. to occur, and the transmission of the data to be delayed).
Lastly, Code Division Multiple Access (CDMA)-based solutions achieve near perfect elimination of ICI using code orthogonality. Neighboring cells use codes that are orthogonal to the codes of the BS considered for communication with a UE. The transmissions are “spread” by an orthogonal code at the BS. At the receiver, the transmissions are “de-spread” to extract the original transmission. ICI originating at a neighbor BS does not significantly correlate (due to the code orthogonality properties), and therefore contributes near-zero noise. CDMA technology may be described as “Rate-1 Frequency-Reuse”, which refers to the fact that there is effectively no ICI between neighboring cells.
While CDMA based solutions are ideal for noise rejection purposes, CDMA systems suffer from other limitations (remaining interference between theoretically orthogonal codes, propagation delay, transmit power adjustment, etc.). Future systems considered in the International Mobile Telecommunications Advanced (IMT-Advanced) framework (i.e., 3GPP LTE Advanced, WiMAX evolution based on IEEE 802.16m, IEEE 802.11 “VHT” (very high throughput), etc.) are expected to be based upon OFDM and OFDMA, due in part to the limitations of CDMA-based systems. Therefore, it is highly desirable that OFDMA based networks approach near “Rate-1 Frequency-Reuse” efficiency.
Accordingly, there is a salient need for improved apparatus and methods which improve OFDMA frequency-reuse by, inter alia, minimizing inter-cell interference. Such apparatus and methods would ideally be implemented within existing radio access network infrastructure (i.e., without requiring significant upgrades or modifications to the network infrastructure), and operate within extant practical limitations such as computational ability and information limitations that would otherwise inhibit “perfect” behavior.
Ideally, such improved methods and apparatus would also be transparent to the UE, so as to allow seamless integration with existing (legacy) UE deployments. They would also be consistent with, and even leverage existing wireless network policies and functional capabilities, and would minimally impact network latency and processing overhead.